1. Field of the Invention
The present invention relates to a method for controlling temperatures in chambers, for example, of environmental testing equipments such as temperature tanks in which predetermined temperatures are to be maintained and temperature/humidity tanks in which predetermined temperatures and humidities are to be maintained. Such chambers are used for testing high and lower temperature resistances, moisture resistance and other characteristics of various electric and electronic products and parts thereof as well as various kinds of material in predetermined temperatures and/or humidities, and are also used for burn-in, screening and others.
2. Description of the Related Art
Temperature control in chambers, e.g., of the environmental testing equipments has been carried out as follows.
Heating means for heating the chamber and cooling means having a constant cooling power for cooling the chamber are used, and an operation of only the heating means is controlled by heat controlling signals which are formed by a temperature controller as a result of PID operation dependent on process temperatures in the chamber, so that the temperature control is performed by balancing a heating rate or power of the heating means and a cooling rate or power of the cooling means.
In the prior art, the powers of the heating means and the cooling means are restricted so as to balance the heating power and the cooling power for the temperature control, and thus cannot exceed the powers required for attaining the balance. Consequently, it is impossible to rapidly increase the chamber temperature which has lowered to a value lower than a lower tolerable deviation value predetermined with respect to an intended set value of the temperature, and it is also impossible to rapidly lower the chamber temperature even when the chamber temperature is higher than an upper tolerable deviation value predetermined with respect to the set value.
It powers larger than the restricted heating and cooling powers were applied to the means in order to reduce a period of time required for raising and lowering the temperature, the temperature control would become extremely instable, which impedes a practical use.
Further, in the temperature controller of the prior art, a large cooling power may be required for rapid discharge of heat when the chamber temperature exceeds the upper tolerable deviation value, for example, due to the heat from an article located in the chamber. However, such signals are not available that activate a cooling source or increase the cooling power thereof in accordance with variation of conditions in the chamber caused by the heating. Therefore, there is a disadvantage that the cooling source can be activated and adjusted only by a manual operation, even if the cooling source having a large capacity is employed.
As a method for overcoming the above disadvantage, it may be contemplated that the rapid increase or decrease of the temperature can be achieved by temperature control means which can supply cooling signals having the PID operation outputs which are symmetrical to the heat controlling signals with respect to a line. In this method, however, the chamber temperature cannot accurately follow the set temperature having a given constant value or a variable value dependent on a time, as can be done in the case previously described, in which the heating power and the cooling power are balanced with each other. Particularly, if the time-dependent set value is to be followed by the chamber temperature, the temperature control characteristics cannot be practically used, for example, in the environmental testing equipments.
Accordingly, it is a first object of the invention to provide a temperature control method for controlling the temperature in the chamber to a constant set value, in which the chamber temperature can be rapidly raised when a current temperature in the chamber deviates from the set value and lowers to or below the lower tolerable deviation value, and the chamber temperature can be rapidly lowered when the chamber temperature is raised to or above the upper tolerable deviation value, for example, due to the heat supplied from an article in the chamber, whereby the chamber temperature can be stably maintained at or near the set value.
A second object of the invention is to provide a temperature control method for controlling the temperature in the chamber to an intended time-dependent set value which varies in accordance with a time, in which the chamber temperature can sufficiently follow the time-dependent set value.